std::experimental:: not_fn

Creates a forwarding call wrapper that returns the complement of the callable object it holds.

Parameters

Return value

Let FD be std:: decay_t < F > and fd be an lvalue of type FD constructed from std:: forward < F > ( f ) .

not_fn returns a forwarding call wrapper fn of unspecified type such that fn ( a1, a2, ..., aN ) is equivalent to ! INVOKE ( fd, a1, ..., aN ) , where INVOKE is the operation described in Callable .

The returned call wrapper is always MoveConstructible , and is CopyConstructible if FD is CopyConstructible .

Remarks

If fd is not Callable , or std:: is_constructible < FD, F > :: value is not true , the behavior is undefined.

Exceptions

Throws no exceptions, unless the construction of fd throws.

Possible implementation

namespace detail {
    template<class F>
    struct not_fn_t {
        F f;
        template<class... Args>
        auto operator()(Args&&... args)
            noexcept(noexcept(!std::invoke(f, std::forward<Args>(args)...)))
            -> decltype(!std::invoke(f, std::forward<Args>(args)...)) {
            return !std::invoke(f, std::forward<Args>(args)...);
        }
 
        // cv-qualified overload for QoI
        template<class... Args>
        auto operator()(Args&&... args) const
            noexcept(noexcept(!std::invoke(f, std::forward<Args>(args)...)))
            -> decltype(!std::invoke(f, std::forward<Args>(args)...)) {
            return !std::invoke(f, std::forward<Args>(args)...);
        }
 
        template<class... Args>
        auto operator()(Args&&... args) volatile
            noexcept(noexcept(!std::invoke(f, std::forward<Args>(args)...)))
            -> decltype(!std::invoke(f, std::forward<Args>(args)...)) {
            return !std::invoke(f, std::forward<Args>(args)...);
        }
        template<class... Args>
        auto operator()(Args&&... args) const volatile
            noexcept(noexcept(!std::invoke(f, std::forward<Args>(args)...)))
            -> decltype(!std::invoke(f, std::forward<Args>(args)...)) {
            return !std::invoke(f, std::forward<Args>(args)...);
        }
    };
}
 
template<class F>
detail::not_fn_t<std::decay_t<F>> not_fn(F&& f) { return { std::forward<F>(f) }; }

Notes

not_fn is intended to replace the C++03-era negators std::not1 and std::not2 .

See also